WDM optical transmission systems carry multiple wavelength channels simultaneously on a single guiding optical line. Their large information capacity is useful in telecommunication applications, in intra-chip and inter-chip photonic networks for advanced high-performance microprocessors and systems for supercomputers, and in various high-bandwidth applications where electronic-photonic hybrid integrated circuits may offer significant advantages, such as in high-resolution, high-sampling-rate analog-to-digital converters, in voice and image data processing, and in biological data processing that is well suited to data parallelism.
With conventional microphotonic filters it is often difficult to achieve hitless tuning and to extend the tunable free spectral range (“FSR”) of the filters to permit addressing a wider optical wavelength range. In addition, replacement of optical components in a live network typically requires temporary shutoff of the WDM network link on which the component is operating, and a down time during replacement.
Interferometric bypass schemes for achieving hitless tuning and the multiplication of the tunable FSR of optical filters are known. These schemes typically employ a class of interferometers termed universally balanced interferometers 10 as shown in FIGS. 1 and 2, though this class of interferometers is not limited to those applications, i.e., hitless filter tuning and FSR multiplication. The schemes are, for successful operation, dependent on having identical splitter 12 and combiner 14 devices, where the splitter 12 splits the input spectrum entering from an input waveguide 24 into two interferometer optical paths or arms 16, 17 and the combiner 14 recombines all signals that remain in the two arms 16, 17 after passing through any optical filters 20 (or other types of embedded optical signal processing devices) into a single output waveguide 18. In the case of hitless tuning/switching, where the splitter 12 and combiner 14 are broadband switches actuated in unison, symmetry implies that the actuation needs to be substantially synchronized. Therefore, in order to scale to very high switching speeds, designs that avoid complex control circuitry and synchronization requirements, while preserving symmetry, are desirable. In the case of the multiplication of the tunable FSR of filters, the splitter 12 and combiner 14 are wavelength filters that need to be identical. Even though the geometry of the universally balanced interferometers 10 is generally first-order insensitive to asymmetries between the splitter 12 and the combiner 14, if the filters are very narrowband, even a small dimensional difference between the two can shift the resonances of the splitter 12 and the combiner 14 apart by more than their bandwidth, and, therefore, significantly compromise their functionality. Thus, an intrinsically more tolerant design is desirable for universally balanced interferometers 10 utilizing splitters 12 and combiners 14 that have narrowband spectral-response features.
Finally, integrated optical circuits are generally inserted into an offline network, and the network then made live. Thus, replacement of integrated optical components, for failure replacement or upgrade, currently requires temporary interruption of network service passing through the device and a down time, since the device ports must be disconnected from the network. Since a WDM network carries a multitude of channels, the majority of which may not be processed by the device being replaced, the device replacement unnecessarily causes a service interruption on a number of express channels. As WDM networks scale to larger numbers of wavelength channels accommodated by a wider spectral range of wavelengths and/or a higher spectral efficiency and utilization of the spectrum, disrupting the majority of channels at a single point may cause greater disturbance to the overall network. Therefore, it is also of interest to find an approach to “hot-swap” an optical signal processing component within the network, that is, to replace it without interrupting service to channels which are physically passing through the optical component being replaced, but are functionally bypassing the component as so-called express channels.